1. Field of the Invention
This invention relates to an insulated wire and a manufacturing method of the insulated wire. In particular, this invention relates to an insulated wire having an insulating layer of a porous member and a manufacturing method of the insulated wire.
2. Description of the Related Art
In an information-processing device, speed increase in transmission signal has been progressed, so that a wire having a foamed insulation formed by extrusion and foam molding of polyethylene or fluorine resin is used as a wire used for the above-mentioned application, so as to place emphasis on low dielectric permittivity.
Recently, since downsizing and density growth of the device have been furthermore progressed, it has been necessary for the wire to be used to have an external diameter of for example, not more than 0.3 mm. Since it has become technically difficult to fabricate the fine wire by the extrusion and foam molding, a method of fabricating a wire having an foamed member by coating and curing an ultraviolet curable resin containing a gas or a foaming agent is proposed. The method is disclosed in, for example, JP-B-3047686, JP-A-07-278333, JP-A-07-272662, JP-A-07-272663, JP-A-07-335053, JP-A-08-17256, JP-A-08-17257, JP-A-07-320506, JP-A-09-102230, JP-A-11-176262, and JP-A-11-297142.
The method is excellent in forming the foamed insulating layer quickly and efficiently, but they have a process of forming the insulation while growing air bubbles, so that they have a problem that growth level of the air bubbles is not easily controlled and variation in foaming level is easily caused. Further, if the variation in foaming level of the insulation occurs, variation in dielectric permittivity of the insulation occurs and simultaneously variation in transmission characteristics of a wire or a cable occurs, so that a problem is caused that signal transmission delay is generated.
Consequently, a method of forming fine foams is proposed. The method is disclosed in, for example, JP-A-2004-2812, JP-B-3963765, and WO2004/048064(pamphlet).
According to the method, the formation of the fine foams is capable of preventing the variation in foaming level of the insulation from occurring.
However, the method disclosed in JP-A-2004-2812 can not be used as an insulator of a wire or a cable, since it has a process of forming fine air bubbles by using a polymer containing a compound degradable with an acid generated by light irradiation and the acid generated erodes a metallic conductor.
Also, the method disclosed in JP-B-3963765 and WO2004/048064(pamphlet) has a problem that plenty of fabricating time is required, so that it is difficult to be used for fabricating a wire or a cable.
Further, a method of fabricating a porous member by using an emulsion is proposed. The method is disclosed in, for example, JP-A-10-36411, JP-A-2004-91569, JP-A-2007-332283, and JP-A-2002-145913. However, the method disclosed in the patent literatures has a problem that it is difficult to be used for a cover of a wire or a cable and simultaneously electrostatic capacitance of the wire varies due to mechanical force, since air bubbles obtained have an interconnected cell structure (mutually combined structure) and collapse deformation is easily caused due to stress such as compression stress, bending stress.